The present application relates to a hologram recording apparatus, hologram recording medium and a hologram recording method.
In recent years, a holographic memory has drawn attentions, which is a recording/reconstructing apparatus that can achieve a high recording density and record/reconstruct record data in a high transfer speed. In a holographic memory, the direction of thickness of a recording medium is also used. In a recording operation, two-dimensional information is handled in one page unit, and interference fringes of reference light and signal light are formed in a hologram (diffraction grating) form to record three-dimensionally in a hologram recording medium based on the page data in accordance with record data. In a reconstructing operation, record data is reconstructed by obtaining diffracted light caused by irradiating reference light to the thus formed hologram (Refer to JP-A-2004-226821 (Patent Document 1) and Nikkei Electronics, issued on Jan. 17, 2005, Pages 106 to 114 (Non-Patent Document 1)).
The recording/reconstruction of a hologram may adopt multiple recording that records holograms in a partially overlapping manner in slightly different areas on which holograms are to be formed on a hologram recording medium. Thus, the capacity for recording record data on the hologram recording medium can be increased (refer to Non-Patent Document 1).
The use of a photopolymer has been proposed as a hologram recording medium for performing hologram recording/reconstruction. However, it is known that a photopolymer has recording and reconstructing characteristic significantly sensitive to a temperature change. More specifically, since the thermal expansion coefficient of a photopolymer is significantly high, the thermal expansion or thermal contract is caused by the change in temperature due to the difference between the temperature of the photopolymer in recording and the temperature of the photopolymer in reconstructing. Then, the diffraction gratings on the photopolymer may rotate and/or the space between diffraction gratings and the refractive index may change, which adversely affects on the recording/reconstructing characteristics. Accordingly, a reconstructing method has been proposed that changes (or compensates) the angle and wavelength of input light in reconstructing in consideration of a change (or difference) in temperature between recording and reconstructing operations.
However, it is known that the characteristic in multiplexing also changes when a temperature changes during a recording operation, in addition to the problem due to thermal expansion, for example. In particular, the diffraction efficiency is largely different in reconstructing after recording since the recording sensitivity largely differs according to the temperature in a case where holograms are formed by using a chemical reaction on a recording layer, which contains a photopolymer, of a hologram recording medium. As a result, at a low temperature, a sufficient diffraction efficiency may not be obtained. At a high temperature, the diffraction efficiency is excessively high, which consumes wasteful M-number (which will be described later). Then, sufficient diffracted light may not be obtained from holograms to be recorded in the future.
A longer period of time may be necessary for multiplexing. Recording without consideration of this point may not exhibit a good recording characteristic. In other words, holograms in a good form may not be formed on a photopolymer. For this reason, it is difficult to exhibit a good reconstructing characteristic, that is, it is difficult to obtain high quality diffracted light from recorded holograms and reconstructing record data accurately even by compensating the angle and wavelength of input light.